An example of a battery pack is described in Japanese Patent Application Publication No. 2009-277471. In the battery pack, multiple battery cells are stacked and electrically connected in series. A battery holder is disposed between the stacked battery cells. End plates are arranged at ends of the stack of battery cells with respect to a stacking direction. The end plates are connected by a binding band. The binding band exerts a binding force to the battery holder in a direction along the stacking direction to fix the stacked battery cells.
Further, an air passage is formed between a surface of the battery cell that is perpendicular to the stacking direction and a surface of the battery holder that is opposed to the surface of the battery cell with respect to the stacking direction. Specifically, the surface of the battery holder has multiple ribs projecting toward the surface of the battery cell. Each of the ribs extends in a horizontal direction, and the ribs are arranged in a vertical direction. Further, an end surface of each rib is in contact with the surface of the battery cell. Thus, multiple air spaces each extending in the horizontal direction are formed in the vertical direction. The air passage is provided by the multiple air spaces. The battery cells are cooled by air flowing in the air passage during its operation.
In such an air passage, since the multiple horizontal air spaces, which are narrow, are arranged in the vertical direction, if the air is unevenly supplied into the air spaces, it is difficult to evenly cool the surface of the battery cell. Further, a boundary layer will be formed in a flow of air between the surface of the battery cell and the surface of the battery holder due to the air spaces being narrow and long. As a result, a heat transfer coefficient between the air and the battery cell will be reduced, degrading cooling performance.